I thought that the 15 minutes came from the minimum time interval of a cheap commercial timer.

I think you got a point there! Being that most growing is indoors, and automated, it totally makes sense. Now why didnt I think of that!!!

Thanks again.

That's correct, the length of the flood cycle needs only to be long enough to fill. Longer than that doesn't help or harm, within reason.

Most systems have an overflow so that the system can be set to run long enough to guarantee a the flood is completed before the pump shuts off, and any excess is simply dumped back into the reservoir until then.

A manual flood like you're doing doesn't need to take 15 minutes, but if you really want to play it safe you could just fill, wait a bit, and the drain.

Don't forget that as your plants get bigger their water demand will get bigger too and you might find yourself struggling to keep the monsters fed.